As is known, extruders are supplied with a mixture of solid loose particles, the mixture generally being made up of polymer powders, for example PVC, and one or more additives, including for example fillers, stabilisers, plasticisers, slips, colorants and others.
Starting from this mixture of loose particles, the mechanical and thermal actions of the extruder enables a continuous, solid and compact manufacture to be produced, such as a tube or a shaped bar.
Each additive present in the initial mixture has a specific function.
Thus, for example, colorants serve to obtain a product of a desired colour, plasticisers for a plastic consistency (rubbery), slips are for improving the workability and facilitating the flowing of the mixture into the inside of the extruder, while stabilisers prevent chemical alteration of the other components of the mixture.
The filler is usually a low-cost inert aggregate which, though having technological functions if added in small percentages (about 2%), is in general added to the mixture in order to reduce the quantity of polymer material to be used.
One of the most commonly-used fillers is calcium carbonate in powder form. Naturally the presence of a filler in the initial mixture reduces the mechanical characteristics of the product obtainable through the following extrusion. Therefore, the percentage of filler is selected on the basis of the quality of the product to be obtained, from a minimum value for high-quality products, for example for realising pressurised tubes, to a maximum value for products requiring lower performance, such as for example pipes for drains and sewers.
The initial mixture is prepared in a turbomixer, which generally comprises an external casing into which the polymer powders and the additives are loaded, and a rotating reel which rotates at high velocity internally of the external casing.
Thanks to the rapid rotation of the reel, the solid loose particles making up the mixture are subjected to both a purely mechanical mixing effect and to an increase in temperature.
The combination of these mechanical and thermal effects are such that inside the turbomixer the particles of the filler and the other additives become uniformly distributed and definitively bonded to the polymer powders, forming with them aggregate particles.
The aggregate particles are very effective in obtaining good results in the following extrusion stage; in the sector many are convinced that the final formulation of the mixture can be obtained only internally of the turbomixer, i.e. that the turbomixer has to be filled with the exact percentages of polymer powders and additives required for the specific product to be extruded.
The fact that many are convinced of the foregoing, however, leads to creating various drawbacks and technical limitations.
A first drawback is that for each different formulation of the mixture, i.e. for each type of product to be extruded, it is necessary to have a respective line of accumulation and distribution downstream of the turbomixer, which not only makes extrusion plants very expensive but also makes them very rigid from the functional point of view, and therefore difficult to manage.
Further drawbacks originate from the phenomenon known as segregation of the additives in the mixture.
It has been found that during the preparation with the turbomixer of PVC powder mixtures and calcium carbonate, when the percentage of calcium carbonate exceeds a threshold value of about 12% of the overall mass, effective aggregation of the two components is no longer achieved, and the excess calcium carbonate remains physically separated from the PVC powders.
It has also been discovered that when the percentage of calcium carbonate is close to the above-mentioned limit value, the two components easily separate later on, for example during storage and/or transport towards the extruder.
The calcium carbonate particles which remain free in the mixture have density and dimensional characteristics which are different with respect to PVC powders, and for this reason they tend to accumulate in some circumscribed zones (sacks) in the overall volume of the mixture, in which the concentration thereof is therefore greater than the required percentage.
This segregation phenomenon is especially manifested during transport of the mixture from the turbomixer towards the extruder, but can also be manifested when the mixture is still inside, for example, storage silos.
The presence of the localised accumulations of calcium carbonate is very risky for the success of the extrusion process, as they can cause considerable defects in the finished products, for example they can cause production of extruded tubes which exhibit holes and/or ruptures.
As it is not possible to act on the composition of the mixture, which has to be rigorously prepared internally of the turbomixer, the only solution at present practicable to prevent these defects is that of over-sizing the products, for example by increasing the thickness of the walls of the tubes to be extruded, with a consequent greater consumption of material and an increase in production costs.
The phenomenon of segregation which has been illustrated herein in relation to a mixture of PVC and calcium carbonate also occurs with mixtures formed by other types of polymer powders and/or other filler substances.
Further, the phenomenon of segregation is also manifested for many other additives which are added to the mixture, which when they go above a relative critical percentage remain free in the mixture and thus tend to gather in circumscribed zones.
The bad dispersion of these additives, among which for example stabilisers, slips and colorants, causes not only a low finished product quality, but also a poor performance of the additives themselves.
Thus, the additives have often to be added to the mixture in greater percentages than those which are in theory necessary, generating a further increase in production costs.